The present invention relates to a transmission for transmitting the power of an engine to propelling and power takeoff systems at the desired number of speed change steps, and more particularly to improvements in speed change mechanism.
Transmissions of agricultural and civil engineering tractors have much higher speed reduction ratios at low speeds than motor vehicles and require many steps of speed changes. Thus transmissions comprising a gear speed change mechanism include very complex speed change gear systems, are very cumbersome to design and make and involve difficulties in accommodating shafts in a limited space.
Tractor transmissions comprising a gear speed change mechanism are generally of the selective mesh type and constant mesh type. In either type, gears or engaging members or clutch pawls are slidingly moved in the axial direction to give desired speed changes so that the mechanism requires an elongated axial length, is subject to damage to gears and gives off a noise. Moreover, each speed change procedure requires engagement and disengagement of the clutch.
More specifically, with the selective mesh type in which a speed change gear on the speed change shaft is slidingly moved to give the desired speed change, the shaft must have a length which is greater by at least the distance of the sliding movement of the gear. The use of gears which must be in the form of a spur gear further gives off a loud noise and involves difficulties in meshing gears due to the difference in peripheral speed between the gears, possibly leading to damage to the gears.
With the constant mesh type, gears on the main shaft are always in mesh with speed change gears on the speed change shaft which is provided with clutch pawls slidable thereon. Accordingly the shaft must be elongated by at least the sliding distance of the clutch pawls. In addition, since the clutch pawls are adapted to engage with or disengage from rotating wheels, the mechanism is subject to damage due to impact.
The gear speed change mechanism invariably requires a clutch which is provided between the engine and the main shaft and which must be disengaged for a speed change. Because the main shaft continues to idly rotate even after the clutch has been disengaged, the speed change action involves a delay especially in the case of the selective mesh type. Tractor transmissions comprising two systems, i.e. a propelling and a power take off system further necessitate a double clutch which is expensive.